1. Field of the Invention
This invention relates to an anti-reflective material for use in photo-lithography in combination with a photoresist for enabling fine processing with high accuracy on irregular substrates and a method for forming a resist pattern using the anti-reflective material.
2. Prior Art
Increased integration and density of modern semiconductor integrated circuits requires increasing the accuracy of pattern dimensions in patterning processes. Fine patterns are generally formed by exposing a photoresist to light in a desired pattern followed by development to produce a resist image, which is known as photo-lithography.
However, if a substrate has an irregular surface, the photoresist layer changes its thickness at steps to induce optical interference. As a result, the resist image becomes low in dimensional accuracy, failing to process a pattern to an accurate size. Also the dimensional accuracy of alignment marks for successive exposure is reduced, leading to a low alignment accuracy.
Several patterning techniques were proposed as having solved the above-mentioned problems associated with substrate surface irregularities, for example, a multi-layer resist technique as disclosed in Japanese Patent Application Kokai (JP-A) No. 10775/1976, an ARC technique (anti-reflective coating beneath resist) as disclosed in JP-A 93448/1984, and an ARCOR technique (anti-reflective coating on resist) as disclosed in JP-A 62520/1987. The multi-layer technique involves forming two or three layers of resist and transferring a pattern to form a resist pattern serving as a mask. Undesirably the multi-layer technique requires a number of steps and is low in production efficiency. Light reflection from an intermediate layer can cause a lowering of dimensional accuracy. The ARC technique is performed by etching an anti-reflective film formed beneath the resist layer. The dimensional accuracy is substantially lost by etching and the extra etching step lowers production efficiency.
In contrast, the ARCOR technique which involves forming a transparent anti-reflective film on a resist layer and peeling the film after exposure is able to form a fine resist pattern to high dimensional and alignment accuracy in a convenient way. By using low index of refraction material, for example, perfluoroalkyl compounds (e.g., perfluoroalkyl polyethers and perfluoroalkylamines) as the anti-reflective film, the ARCOR technique minimizes reflection light at the resist layer/anti-reflective film interface, thereby improving the dimensional accuracy of a resist image.
However, since the perfluoroalkyl compounds have low compatibility with organic materials, they must be diluted with such diluents as Freon solvents in order to control the thickness of a coating film. The use of Freon is now undesirable from the standpoint of environmental protection. In addition, the perfluoroalkyl compounds form less uniform films whose performance is less satisfactory as an anti-reflective film. Since the anti-reflective film of perfluoroalkyl compound must be peeled off with Freon prior to development of the photoresist, an extra means for peeling the anti-reflective film must be added to the existing system. These combined with the cost of Freon solvent are the demerits of the ARCOR technique from a practical aspect.